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<br />the IOO-ye:!r event, as well as for a maximum rekasc rate of 5,000 cf, from Chatlid<J Dam. The
<br />modd was calibrated b}' adjusting Mannin{s "n" valu.:s and using high water mark information
<br />from flows t.1ken during June of 1997 by the UD&FCD. The calibration run was approvcd by the
<br />proj.:ctsponsorsand th.: r<:sulb incorporated into the final model. Chann.:l crass-section data was
<br />t:ll.:cn from topographic mapping developed for the project This data was provided in AutoCAD
<br />RI3 format as digital. three-dimensional mapping at I-foOl contours, Additional field survey data
<br />WaS colkcted at each cross-s.;etilln to determine the profile of the channd bottom beneath tll\: water
<br />surface. Th.: swve~'ed data was used in lieu of the mapping at each cross-section for the porrion thm
<br />was below th.: wat.:r surface when the mapping was l1o'Wn,
<br />
<br />total of 10 sheets arc provided depicting the 100-year 1l000ldplain, IOO-year and 5,OQ() cfs water
<br />surface profiles, An index of map, heelS is also provided for reference.
<br />
<br />Geomelric measurements, low chord, and invert ekvalions at bridges were lkld ,urveyed and
<br />provided 10 th.: proj~'\:t sponsors by Accurate Engineering, Inc. Photographs were taken m each
<br />bridge crossing and a skcteh of each was prepared showing the appropriatc dimerlsions. This
<br />detailed data, in cOrljunction with the mapping, was us~-d to enter the required parameters to model
<br />thcbridges.
<br />
<br />Profiles of the left and right hank devations were determin.:d from the top..)graphie mapping and
<br />were plotted on the profile sheets. The top of bank was takcn to bc the point at which lhechannel
<br />sideslope ehallged 10 a much Oanel grade, Wh.:re such a grade break was not obvious. the inside
<br />edge of the pedestrian trail was sometimes used, Bank profiles were not shown dO\\l1stream of
<br />Oxford Avenue since the channelization ended at that location. These profiles provide a grapbkaI
<br />representation of th.: estimated available freeboard in the ehannd. The computed water surface
<br />elevation, cast and wesl bank e1evatiorls, and minimum charlnel trecboard area arc presented in Table
<br />2,1 for the 100-year event. Thc minimum channel freeboard is approximately 1.4 feet near the
<br />conl1uenceofBig Dry Creek. None ofthc bridges in the study reaeh arc surcharged bylhe 100-year
<br />evenl.
<br />
<br />Photographs were t:ll.:en to document the existing crmnnel and bank conditions f'lr determining
<br />roughnesscoeffieients for use in the hydraulic analysis. This supplcmentary material is located ina
<br />separate report enlitl.:d Manning',. ",," Vallie Reportjor the Sauth Plaue Rlwr Chann,", Capacity
<br />(Rdcrence 2). The Manning's "n" values utilizcd in the hydraulic anal)'sis were 0025 for the
<br />channel bOllom and0050 for lhe channcl banks.
<br />
<br />Forcomparisonpurposcs.thc IOO-year water surface and channcl invcrt profiles generated by the
<br />Corps for the original ehanncl dcsign upstream of Union Avenue was superimposed on the profile
<br />sheets. The water surface c1evations were taken from the Corps HEC-2 model and adjusted 10 the
<br />profiles based on locations of existing bridge and roadway crossing structures. [n addilion. the
<br />vertical datum oflhe Corps watcr surface and ehanncl invert profiles were raised by 2.4 f~>t:t which is
<br />the average datum difference between the 1929 map datum that the Corps uS<.-d and the 1988 datum
<br />on which lh.: new mapping was prepared. Generally, the profiles indicate that the river bottom has
<br />degraded downwaro (ie. the channel section deepened) since the original construction., This is
<br />consislentwith survey dala whiehhasbeenreeordedovcrthepastseveral}'earsatvariou~localions
<br />on the river in and adjacent tu this ,tud~ reach. This survey data is measured cach ycar by lhc
<br />L'D&FCD.
<br />
<br />The h}'draulie model for this study begins at Hampden Avenue and proceeds upstream 25,590 fed to
<br />lhe Brown Ditch Diversion at Coal MlIle Road (txtcnded). Stationing for the mood was slart~.J at
<br />16~5+26 to coincide with the report South Plall~ River Flood lfa:;urd Area Delineurion, Sand Creek
<br />10 Oxford A.'enuc (Reference 3). The eh,urnc1 centerline between Hampden and Oxf<1fd '.vas
<br />e<1rrelatedtothecenterlinedepicledintheaooverefcrencedrepol1toattempttokecpapproximatdy
<br />the same stationing up to Oxford Avenue Cpstream ufOxford Avenue, the channel centerline and
<br />stationing were taken midway betwcen the edge of water lines as depicted in the tOp(lg"~phie
<br />mappingpruvided bytheprojeet~ponsors.
<br />
<br />A total (If 75 eross-scctions were ,anlpled. with an average spacing of about 350 feet. Tbe cros~-
<br />S<.'\:tions are identified according l() their rivtr swtiull ...,..l ..." ,hu,",'i VLI t},<: pi",; ar,,j pr0fik .ih~~ti
<br />included in this repurt. The starting watcr surface e1evation for the 5,000 cfsevent wa,dctemlined
<br />using the ,Iop<::-areaoption with an initial energy slope of 0.0005 and ancstimated walersurfaee
<br />c1evation at Hampden Avenue. The starting water surface elevation al Hampden Avenue. as well as
<br />the 100-year now rates downstream of Oxford Avenue were taken from the South Platte River
<br />FHAD (Rcfcrellee I). All of Ih.: remaining l10ws for the 100-year event were taken from the model
<br />used by the CDE for their channel design. The location or the eha~e in !lows were eorrcl,lled ...,.jth
<br />the eross-~eetiuns u>cd in this study. The localions and values at each change in 100-y'car flo\\'S are
<br />Sh\1Wnintheprofi\cs.
<br />
<br />2.2 Results
<br />
<br />The results of the hydraulic analysis arc ,huwn on drawings and in tables in this reporr. Drawings
<br />indudcplanv\c\\'mapsandprofiles.eachindicatingl1oodelevationsand cross-s.;etion locations. A
<br />
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